1. Field of the Invention
This invention relates to apparatus for protecting electric power distribution networks, and more particularly, to a network protector with a drive circuit for controlling a trip actuator powered by network voltage which can vary over a wide range.
2. Background Information
Electric power distribution networks which supply power to a specified area such as, for example, a section of the city, an industrial complex or a large building, are fed at multiple points through feeders which each include a network protector. A network protector is a circuit breaker adapted to trip and open the feeder upon detection of reverse power flow, that is, power flowing through the feeder out of the network rather than into the network. Typically, overcurrent protection is provided by other devices such as fuses in series with the network protector.
The network protector is energized by the voltage on the network at the point of connection of the network protector. Standards require that the network protector be able to trip at network voltages as low as 7% of rated voltage. Conventional network protectors have been able to accommodate sufficiently large trip actuator coils having an impedance capable of limiting coil current at the high end of the voltage range while still being able to operate at the low end. A new design of network protector utilizes a circuit breaker which is very compact, and accordingly, has trip coils which are smaller. The problem is exacerbated by the requirements of some users that the network protector be able to operate at higher than rated voltage, by as much as 50%. This extended range of operation, when combined with the smaller physical size of newer trip coils presents a formidable challenge to providing a trip actuator having sufficient impedance to limit coil current at the high voltages while still being able to operate at the low limit of network voltage. It must also be taken into consideration that network protectors are called upon to trip quite frequently and therefore must have significant cycle life.
There is a need, therefore, for an improved network protector and trip actuator therefor which can operate over the full range of required trip voltages without burning out the coil.
This object and others are satisfied by the invention which is directed to a network protector having separable contacts and an operating mechanism opening the separable contacts when actuated. The operating mechanism is actuated by energization of a trip actuator coil. A control relay generates a trip signal in response to detection of power flow out of the network. A coil drive circuit responsive to the trip signal and powered by the voltage in the protected network energizes the coil. This coil drive circuit incorporates a current limiter limiting energizing current to the coil to a specified value regardless of the wide variation in voltage in the protected network.
The coil drive circuit includes an electronic switch connected in series with the coil and a control circuit which turns the switch on when the coil current is less than the specified value and turns the electronic switch off when the coil current exceeds the specified value. The control circuit includes a current detector generating a current detector signal proportional to the coil current, a reference signal generator generating a reference signal proportional to the specified coil current and a comparator which compares the detector signal to the reference signal. The electronic switch is turned on when the current detector signal is less than the reference signal and is turned off when the current detector signal becomes greater than the reference signal.
The control circuit further includes a latch latching the comparator to turn the electronic switch off once the current becomes greater than the reference signal.
The coil drive circuit can include a voltage generator which generates a coil supply voltage proportional to the voltage in the network. This coil supply voltage is applied across the coil and the electronic switch. The control relay includes trip contacts which connect this voltage generator to the network to energize the coil upon detection of the reverse power flow condition. The reference signal is generated from the output of the voltage generator as a constant value signal proportional to the selected coil current.
The invention also embraces a drive circuit for a trip actuator coil in a network protector providing protection to an electric power distribution network over a wide range of network voltages. This drive circuit comprises a voltage source selectively powered by the network and generating a coil supply voltage proportional to the network voltage when powered. An electronic switch is connected in series with the trip actuator coil and the voltage source so that the coil supply voltage is applied across the trip actuator coil with the electronic switch turned on. The drive circuit further includes a control circuit energized by the voltage source which turns the electronic switch on when energized by the voltage source and turns the electronic switch off when current through the trip actuator coil exceeds a specified value. The control circuit includes a current detector generating a current detector signal proportional to the coil current and a reference signal generator generating a reference signal proportional to the specified value of coil current. A comparator compares the current detector signal to the reference signal and turns the electronic switch on as long as the current detector signal is smaller than the reference signal. When the current detector signal becomes greater than the reference signal, the electronic switch is turned off.